Polymer powders redispersible in water—(re)dispersion powders—are obtained by drying the corresponding aqueous polymer dispersions in the presence of a drying aid (in general a protective colloid). Owing to the proportion of protective colloid, firstly irreversible sticking together of the polymer particles is prevented during the drying process since the polymer particles are surrounded by the water-soluble protective colloid particles. Secondly, this protective colloid matrix, which dissolves again on dispersing of the polymer powder in water, ensures that the polymer particles are present again with the particle size of the starting dispersion in the aqueous dispersion.
Such dispersion powders are used in all kinds of applications, for example in coating materials, in adhesives for a very wide range of substrates. Most widespread is the use thereof in construction chemistry products, frequently in combination with hydraulic binders. Examples of these are construction adhesives, in particular tile adhesives, renders and mortar materials, paints and filling compounds, leveling compounds, composite heat insulation systems and joint mortar. The advantage of the dispersion powders lies in particular in the possibility of using them in prepared, storable dry mixes, optionally together with hydraulic binders, such as cement, and making them ready for use only immediately before use by addition of water. Such dry mixes can be more easily transported than pasty systems (without a proportion of water) and have advantages during storage.
The use of dispersion powders serves in particular for improving the mechanical strength and the adhesion of mortar materials modified therewith on film formation of the polymer particles. It is known that polymer films can be crosslinked for further improvement of the mechanical strength.
U.S. Pat. No. 4,028,294 discloses the preparation of crosslinkable polymer latices by using an epoxy resin as a mixture with monomer in the emulsion polymerization. Moreover, to obtain a crosslinkable system, polymerization is also effected in the presence of epoxy-reactive comonomers which carry groups crosslinking with the epoxy resin. It is alleged that no premature crosslinking occurs at the temperatures chosen for the polymerization. The preparation of polymer powder compositions redispersible in water is not described. U.S. Pat. No. 4,108,819 describes the copolymerization of vinyl acetate and ethylene in the presence of an epoxy resin. For preventing premature crosslinking, the pH is kept at pH 3 to pH 7 during the polymerization. An amino curing agent is then added to the prepared dispersion. A disadvantage in the case of these processes is the danger of premature crosslinking since both epoxide groups and amino curing agent are present in the aqueous dispersion and therefore at least partly crosslink during storage. The preparation of polymer powder compositions redispersible in water is not described. In U.S. Pat. No. 6,235,811 B1, aqueous dispersions of epoxy-modified vinyl acetate (co)polymers are prepared by a procedure in which a vinyl acetate (co)polymer latex is mixed with the liquid epoxy resin or an aqueous solution of the epoxy resin, and liquid isophoronediamine is then added. Here too, epoxy resin and amine crosslinking agent are present side by side in the liquid phase, which leads to premature crosslinking on prolonged storage. DE 2601200 describes an aqueous vinyl acetate-ethylene copolymer dispersion which was obtained by polymerization in the presence of an epoxy resin and to which an amine crosslinking agent is added. Since the reactive components are present side by side in the aqueous phase, such systems tend to premature crosslinking. In U.S. Pat. No. 4,690,988, polymer-modified, vinylized epoxy resins are obtained by first reacting a polyepoxide with an ethylenically unsaturated carboxylic acid and polymerizing further monomers (vinyl-functional chain-extending monomers) in the presence of the vinylized epoxy resin. The crosslinking of the polymers obtained thereby is effected by addition of (hydro)peroxides. Premature crosslinking is difficult to rule out in the case of such systems.
A crosslinkable dispersion powder is obtained, according to US 2001/0024644 A1, by copolymerization of ethylenically unsaturated monomers and epoxide-functional monomers in an aqueous medium, drying of the dispersion and subsequent addition of polyepoxide. A disadvantage is that the crosslinking efficiency here is relatively low. EP 0896029 A1 describes a process in which ethylenically unsaturated monomers are copolymerized with comonomers which contain crosslinkable groups, the dispersion is dried and solid particles which carry functional groups which can crosslink with the crosslinkable groups of the copolymer are then admixed. EP-A 723975 A1 discloses a dispersion powder which is obtained by copolymerization of styrene and/or (meth)acrylates with epoxide-functional comonomers. A disadvantage in the case of these processes is the danger of premature crosslinking during polymerization in the presence of crosslinkable comonomers. EP 721004 A2 discloses polymer powder compositions which contain polymer powder and pulverulent, crosslinking components, which are obtained by applying the respective liquids to a pulverulent carrier. The pulverulent components are then mixed with one another. A disadvantage there is the handling of such a 3-component system and the poor redispersibility of such mixtures.